Those who have been on a similar journey to the one Russell Davies is about to begin have plenty of advice and tips for the Wieden & Kennedy senior manager as he moves on to become Nike's global planning chief.
Initially, he may feel a sense of power that's almost scary, they say.
He'll find he can influence the direction of a world-famous brand in a way he could never do while working in an agency.
What's more, his background will give him an instant empathy with his agencies. They'll accept him as one of their own and take comfort that he understands the importance of good briefs and the way the creative process works.
He'll also draw strength from the knowledge that comes from being a poacher-turned-gamekeeper.
As one agency chief used to dealing with ad people who have crossed the divide points out: "You can't pull the wool over their eyes."
But an adman-turned-client needs to beware. You must resist the temptation to spend too long with your agencies. Advertising may form only a small part of your responsibilities and there's a risk you'll neglect more pressing concerns. Colleagues may come to regard you as "one of them" rather than "one of us".
Moreover, you'll find your power is more limited than you thought. You may have been top dog in your agency but now you're a small cog in a big wheel with procurement people breathing down your neck.
Robert Bean, who moved from WCRS to be the head of advertising for BT's consumer arm, says: "Moving client-side is like having cold water splashed in your face."
Chris Macleod, the former CDP chief executive who became the UK and European marketing director for Papa John's, the home-delivery pizza chain, agrees: "You'll find the job is a hell of a lot broader than what you've been used to."
Nevertheless, the expectation is that the number of senior agency managers joining client companies will grow from a trickle to a flood.
Improved financial rewards are helping to fuel the trend. The time has gone when agency managers earned significantly higher salaries than their client counterparts.
But the prospect of fatter pay cheques isn't the only reason. As supergroups such as WPP have grown, so the ad industry has become more corporate, making the divide between agency and client company easier to leap.
Furthermore, big client-company jobs have become much more attractive to top agency executives. Many client jobs extend beyond pure marketing and involve the co-ordination of a range of communication specialists across the globe.
"Senior agency people are used to handling clients' disparate communication requirements and so are well equipped to take these roles," Chris Thomas, the Proximity London chief executive, says.
However, some say the gulf between client and agency cultures can't be bridged easily. Rupert Howell, the McCann Erickson UK chairman, suggests that agency staffers seeking the greener grass on the other side of the fence tend not to stay long.
"There are fundamental differences between client and agency people," he claims. "Clients relish building a depth of knowledge by working on a single brand, whereas agency people relish applying their knowledge across many brands. It's not that they're flaky, but they tend to have short attention spans."
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FORMER AD MAN - Neil Simpson, global marketing communications director, Vodafone (former account executive at Bartle Bogle Hegarty)
"Ex-ad people make tough clients because they can cut through the crap and the red tape that comes with all the agency bureaucracy there is. This means they are well equipped to deliver the best environment and processes.
"Often, the most difficult thing is to nurture creative ideas on the client side, but an ex-account man's skills are very relevant to achieving this.
"In the future, we may see more reverse moves happening. Already there seems to be a growing trend of clients moving back into the agency world, which will only help sharpen the communications companies of the future."
CLIENT CHIEF EXECUTIVE - Chris Moss, chief executive, The Number
"Agency people make great clients because when they step over the water they are a poacher-turned-gamekeeper and therefore understand what makes an agency tick. So when the agency is getting a bit precious they can bring it back down to earth.
"The client/agency relationship is built around trust and understanding, and former agency people tend to have a lot better realisation that this relationship is the key to the whole thing.
"In my view, clients are not the best clients if they have not done time on the agency side."
HEADHUNTER - Isabel Bird, chief executive, Bird & Co
"If a global company wants a traditional marketing director, it will not go to an agency because ad people view advertising and brand strategy as the whole job. But if it is looking for a strategic expert and an innovator who can help position its global brand, then an ad agency is exactly the right place to recruit from.
"I think these moves are easier when they are at chief executive level, because then you are orchestrating a group of people to make a business more successful and you don't have to be so functionally oriented.
"We always try to shortlist an ad person for the position of a non-executive director at a FTSE-100 company."
AGENCY BOSS - Tim Delaney, chairman, Leagas Delaney
"Should ad people become clients? Why not? But I suspect that it's quite a culture shock for most of them. Agencies are, rightly, informal, while client companies are a lot less so. Agencies have a roster of clients, which can make every day and every hour different. Client companies, by comparison have a narrower focus, sometimes to the point of obsession.
"Perhaps the biggest difference is that, in reality, communications for clients are but one of the issues that impact on their business, whereas advertising and the people within it derive their energy and ideas from the belief that the world revolves around their next campaign."